Basic Knowledge, Circuit and Application of Operational Amplifier(op amp)

What is an op amp?

Operational amplifiers (referred to as op amps) are high-gain voltage amplifiers with DC coupling, differential mode input and usually single-ended output, because they were mainly used in addition, multiplication and other operational circuits at the beginning. An ideal operational amplifier must have the following characteristics: infinite input impedance, equal to zero output impedance, infinite open loop gain, infinite common mode rejection ratio(CMRR) , infinite bandwidth. The most basic operational amplifier is shown below. Operational amplifier modules generally include positive input (OP_P), negative input (OP_N) and output (OP_O).

The most basic op amp

Two basic concepts of op amp

“Virtual short” and “virtual broken” are two basic concepts of operational amplifiers. When analyzing linear application circuits of operational amplifiers, the analysis process of application circuits can be simplified. Operational circuits formed by operational amplifiers require a certain functional relationship between input and output, so both conclusions can be applied.

“virtual short”

The voltage amplification of the operational amplifier is very large. Usually, the open-loop voltage amplification of the general-purpose operational amplifier is above 80dB, and the output voltage of the op amp is limited, generally between 10 V and 14 V. Therefore, the differential mode input voltage of the op amp is less than 1mV, and the potential of the two input terminals is approximately equal, which is equivalent to “short circuit”. The greater the open-loop voltage magnification, the closer the potentials of the two input terminals are to equal.

“Virtual short” means that when the op amp is in a linear state, the two inputs can be considered equal potential. This characteristic is called false short circuit, or virtual short for short. Obviously, these two input terminals can’t be truly short-circuited.

“virtual broken”

Because of the differential mode input resistance of operation amplifier is very big, general-purpose operational amplifier input resistance is usually more than 1 m Ω. Therefore, the current flowing into the op amp input is usually less than 1uA and far less than the current of the external circuit at the input. Thus, the two inputs to an op amp are generally considered open, with the greater the input resistance, the closer the two inputs are from the path.

“Virtual broken” means that when the analysis op amp is in a linear state, the two inputs can be treated as an equivalent open circuit. This feature is called false open circuit, or virtual break for short. Obviously, these two input terminals can’t really be disconnected.

Two basic circuits of op amp

Negative feedback circuit

Negative feedback circuit of op amp

The polarity of the feedback signal and the input signal are opposite or the direction of change is opposite (inverted), the result of superposition will weaken the net input signal. This kind of feedback is called a negative feedback amplifier circuit. The sampling of negative feedback generally uses current sampling or voltage sampling. The use of negative feedback makes the closed-loop gain of the amplifier stabilized, eliminating the influence of the open-loop gain. Negative feedback also affects the amplifier’s input and output impedance. Voltage mixing increases the input impedance, current mixing decreases the input impedance; current sampling increases the output impedance, and voltage sampling decreases the output impedance. The use of negative feedback can also greatly reduce the distortion generated by the amplifier in a steady state, and can reduce various interference levels within the amplifier.

Positive feedback circuit

Positive feedback circuit of op amp

The polarity of the feedback signal is the same as the polarity of the input signal of the system, thereby enhancing the net input signal of the system, which is called positive feedback mode. In the electronic amplifier circuit, the use of positive feedback can increase the amplifier gain and increase the amplifier’s frequency selectivity to generate useful periodic oscillation signals.

Operational amplifier application

Generally, using negative feedback can ensure the stable operation of the circuit. And the use of positive feedback can be applied to systems with oscillating signals. The operational amplifier has the functions of addition, subtraction, proportional amplification, integral differentiation, etc. There are many operational amplifier circuits that cover most of the main analog functions required. Therefore, operational amplifiers have become the workhorse for analog electronics designers, which have a wide range of uses in the electronics industry, especially general purpose op amps. They can be used as precision AC and DC amplifiers, active filters, oscillators and voltage comparators when connected to appropriate feedback networks.

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October 2020